MedDRA Structure

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Chapter: Pharmacovigilance: Medical Dictionary for Regulatory Activities (MedDRA)

MedDRA is supplied as flat ASCII files. These files are linked and arranged in a hierarchical matrix.


MedDRA is supplied as flat ASCII files. These files are linked and arranged in a hierarchical matrix. Each MedDRA term is presented as words and also comprises a non-logical 8-number code. The terms are organised within 5 hierarchical levels: Lowest Level Terms (LLT); Preferred Terms (PT); High Level Terms (HLT); High Level Group Terms (HLGT); and System Organ Class (SOC) (Figure 13.1). Concep-tually, it can also be considered that the terms are arranged into 26 vertical axes, each represented by an SOC.

Lowest Level Terms – around 60 000 in number at the time of writing – are at the bottom of the hier-archy and consist of synonyms, lexical variants, and other similar representations of specified medical or associated conditions. These terms are intended for entry onto a database for purposes of ‘coding’ the data. The large number of available LLTs provides a high degree of probability that the words used by the individual – for example, a doctor reporting an adverse reaction (the verbatim or ‘reported’ term) – will be represented in MedDRA as an identical, or very similar, LLT. However, some LLTs are referred to as ‘non-current’. These are obsolete, ambiguous or mis-spelt terms, sometimes inherited from other terminologies, or ones that breach MedDRA’s rules in some way, or that are in some other way unacceptable for routine use. They are retained in MedDRA to facil-itate conversion of historical coded data but should not be used for coding new data. MedDRA terms are never deleted from the terminology, although terms may be demoted to the lowest level and made non-current.

Similar LLTs are linked to the same PT, of which there are of the order of 16 000. An example is shown in Table 13.1. Each PT is also duplicated as an LLT. The PT level is that favoured for use in case retrieval and data presentation, each PT ostensibly representing a unique medical concept (although in reality there may be overlap). PTs associated with similar medical conditions are in turn grouped under some 1 600 HLTs (approximately). Examples of PTs grouped under an HLT are shown in Table 13.2. HLTs are grouped as clusters under some 300 or so HLGTs, an example of which is shown in Table 13.3. HLGTs in turn are distributed among 26 SOCs, as shown in Tables 13.4 and 13.5 respectively.

These hierarchical groupings help bring together similar medical conditions for purposes of case-finding and presentation. Thus the HLTs and HLGTs may help to subdivide large tables of aggregate data, as shown in Table 13.6.

As with some other terminologies and classifica-tions (e.g., WHO-ART or the International Classifi-cations of Diseases), MedDRA is referred to as being ‘multiaxial’. This means that a PT (with its subor-dinate LLTs) may be represented in more than one SOC. In this case, MedDRA designates one SOC as being ‘primary’, for purposes of data presentation. The other locations (up to 4) of the PT are referred to as ‘secondary’ locations.

An example of the multiaxial structure of MedDRA is shown in Table 13.7. A problem arises for some users of MedDRA because their database systems do not adequately handle the MedDRA data model. Hence, they may be unable to utilise the secondary location of terms. This is unfortunate, as secondary locations facilitate finding all cases relevant to a particular medical condition. Thus, for example, if there is interest in finding all reports of ventricu-lar arrhythmias, it is helpful that cases of Sudden death (primary location of the PT is in the General disorders SOC) would be retrieved in a search of the Cardiac disorders SOC under the HLT Ventric-ular arrhythmias and cardiac arrest, as the term has a secondary location there – if the database system functions adequately. This will be considered further under Database Searches below.

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